Yes, 21 is LIGHTNING ( electrical discharge) minus N ( the "essential" or middle letter of potential) making LIGHTING.
As for your other tricky clues, 1, 9 and 10 are all pretty unusual words to be honest, 18 less so.
1 is a reversal of a three letter word meaning disdain followed by K for king.
9 is another three letter reversed word surrounding I (one ). For can, think bathroom...
The definition for 10 is the first three words, then look at the next eight words carefully...
For 18, jazz here indicates an anagram
Good luck!